Our org relies heavily on Dropbox
for shared document collections, but most of the team members are Mac users, and iCloud's seamless file syncing between a single user's iOS MacOS devices has been a really useful addition to the mix. Most of us use iCloud to store and archive files created by compatible applications, then move things to Dropbox
if they're "done" and need to be accessed by multiple team members.
In a few situations, we've used the collaborative authoring tools that Apple's free productivity apps provide (multiple users working on the same Keynote file in the leadup to a large meeting, for example).